Measuring means for measuring the concentration of gas components of the respiratory flow of a patient are known from clinical practice. They are used here especially in the medical method of capnometry, in which the carbon dioxide content in the air expired by a patient is measured and monitored. The respiration can be adapted to the patient relatively well on the basis of the carbon dioxide concentration. It is also possible to recognize early whether an endotracheal tube is in the correct position and whether the patient's metabolic situation is correct. The latter point makes it possible to perform interventions early in case of certain complications, such as malignant hyperthermia, or it yields a reference value for the effectiveness of resuscitation.
The mainstream method, in which a measuring gas sample holder is provided in a respiratory flow between the respiratory tract of a patient and a respirator, is preferably used to measure the concentration of gas components of the respiratory flow. An infrared measuring method, which involves radiation through the tidal volume flow, as it is disclosed, for example, in DE 19 520 488 C1, in which the concentration-dependent absorption of the infrared radiation emitted by an infrared radiation source is detected after passing through the respiratory flow in the measuring gas sample holder for a wavelength characteristic of a certain gas, such as CO2, is used for this. The concentration of the gas determined can be determined by calculation from the measured signal received. The measuring gas sample holder has for this two opposite windows, which are arranged at right angles to the direction of flow and through which infrared light is sent.
A measuring gas sample holder for measuring the concentration of gas components of the breathing air, in which the measuring gas sample holder housing and the measuring gas sample holder windows, through which an infrared light is sent, are manufactured separately, is known from WO 2004/096043 A1. The measuring gas sample holder windows are manufactured according to an injection molding process and are subsequently connected to the measuring gas sample holder housing according to a suitable bonding method. A defined distance between the measuring gas sample holder windows is essential for the calculation of the concentration of a certain gas. The distance defines an inner volume of the measuring gas sample holder, which volume affects the calculation of the concentration of the gas components of the breathing air. Due to the bonding in the measuring gas sample holder windows, the distance may deviate from a defined value and inaccuracies may consequently develop in the calculated concentration values.
Furthermore, the measuring gas sample holder windows have a very small wall thickness. Even though a small wall thickness of the measuring gas sample holder windows is advantageous for the transmission characteristic for the infrared light, motion of the windows, which may lead to a distortion of the measured values, may occur as a consequence of a sudden change in pressure in the respiration circuit because of the small wall thickness, because the distance between the measuring gas sample holder windows varies. The measuring gas sample holder windows are provided with an outwardly arched side, which are fixed to the housing in the direction of the interior of the measuring gas sample holder. High-precision manufacture is necessary for manufacturing the measuring gas sample holder in order to avoid unevenness as a consequence of a projection of the measuring gas sample holder windows in the area of a duct in the interior of the measuring gas sample holder, which said duct is formed by the measuring gas sample holder windows and through which the patient's breathing air flows.
Furthermore, high requirements are imposed on the adhesive concerning biocompatibility.
A gas-measuring device operating according to the principle of infrared absorption for the continuous measurement of the CO2 content in breathing gases, in which a sample holder tube consisting of a plastic transparent to infrared light is arranged replaceably in a bracket surrounding the sample holder tube, is known from U.S. Pat. No. 4,011,859 A.
WO 2004/081612 A describes a measuring gas sample holder, in which a ray path between a radiation source and an infrared radiation detector has a surface reflecting infrared light.